1. Field of the Invention
The present invention relates to a semiconductor device having a lateral transistor.
2. Description of the Related Art
As a high-voltage switching element, a lateral transistor is used which has a structure for alleviating electricity field intensity between a gate electrode and a drain region. Laterally Diffused MOS (LDMOS) transistors and High Electron Mobility Transistor (HEMT) are known as a kind of the lateral transistor. For example, when an AC-DC converter is made of a single chip, a high-voltage LDMOS transistor is used as a power MOS transistor.
In order to suppress fluctuation of electrical properties of a LDMOS transistor such as voltage resistance and on resistance, a structure has been proposed in which a field plate is located on an insulation film between a gate electrode and a drain region. The objective of the structure is to realize a highly reliable high-voltage MOS transistor that is not affected by external charge by covering an insulation film between a gate electrode and a drain region with a floating conductive body and locating a field plate thereon.
There is a problem in a LDMOS transistor in that, although a channel is formed in a semiconductor layer beneath a gate electrode in an on state and current flows between a drain electrode and a source electrode, on resistance becomes high due to resistance of the semiconductor layer between a gate electrode and a drain region.